1. Technical Field
The present invention relates to shot peening processes for providing high-strength steel members for use in various high-strength parts and dies, particularly to a shot peening process for providing an unprecedentedly high compressive residual stress of 2,200 MPa or more.
2. Background Art
In order to prolong the service lives of various high-strength parts and dies, the surfaces of the high-strength parts and dies have been strengthened by various heat treatment processes such as induction hardening, carburizing, carbonitriding, and nitriding and shot peening processes under various conditions. For example, JP2009-131912A (Patent Document 1) discloses the surface hardening of a steel material by eutectoid vacuum carburizing and then shot peening of the hardened material with a shot having a higher hardness to apply a high compressive residual stress of 1,800 MPa or more.
Recently, various high-strength parts and dies have been used in even severer environments, and there has been an urgent need for the development of a technology that allows for further strengthening of the components and the application of a higher compressive residual stress. Unfortunately, conventional combinations of shots and workpieces to be shot-peened result in a compressive residual stress below 2,200 MPa. This is due to the insufficient hardness of both the shot and the workpieces. The surfaces of materials subjected to general shot peening processes also soften noticeably when they are used in applications involving a temperature increase, such as gears and molds.